RFA signings are exceedingly rare. Waiting until as late as April 24 to know whether or not you have a 3d round pick is silly unless there's a good reason for it. Maybe Sanders and the Patriots have been negotiating for 3 weeks. Doubtful. If someone else was going to sign him to an offer sheet, it would have happened, so he's not exactly high in the leverage department.
Anything is possible. My read on the situation is that it was something the Patriots considered three weeks ago, and it didn't happen. The only thing that makes me wonder is that I don't think they are going to wait for the draft with $10 million in cap space, so I expect they still intend to do something. But I would be surprised if anyone really thinks it's Sanders, other than the people in this thread.